Ok, first, there are several types of polymers. There are thermoplastics, which means you heat them up and they melt, cool them down and they solidify. Then there are thermoset compounds. They are usually more fluid at first, but once they are heated and or "cured", they are " set " and no longer melt. Many overlaps between the two, as well as latex and a bunch of other stuff.
There are standard definitions of runner and plastics types. EPDM, NR(natural rubber), polyisoprene, neoprene, polyvinylchloride, polyethylene, polypropylene, and polystyrene. Thats just the start of the list. But with almost any polymer, it will degrade over time. Usually, the designer will make a trade off. They will use a less expensive polymer and increase tensile strength or other properties with additives. The same with degradation. As an example, If the polymer has double bonds subject to ozone attack, an additive like wax might be added. It will eventually bloom out of the compound and protect it from ozone. That is a very ancient example, but you get the idea.
As for the additives or polymers used in video games, sorry, can't help. My background is pharmaceutical/medical devices.
As for polymers with lifespans of 50 years. The consumables typically had a shelf life of 2 years. The parts for the instrument is themselves were much longer. But when you buy an analytical instrument from a major instrument company, they also will work hard to sell a service package. This typically is 10% annually of the instrument cost. In something like a high pressure liquid chromatograph,.parts wear out often. They have a lot of moving parts. But plastic parts are usually pretty reliable( knock on wood).